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### Topic: 1st order rate constant question  (Read 6008 times)

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#### kmhard1

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##### 1st order rate constant question
« on: July 15, 2008, 01:24:59 PM »
Someone told me it is better to determine the 1st order rate constant, when measuring reactant A concentration, from ln-([A] - [A]o) vs. t, forcing the intercept to 0.  He said it is wrong to determine from ln[A] vs t because this assumes [A]o is a variable and should not be.  Is he correct?

#### Borek

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##### Re: 1st order rate constant question
« Reply #1 on: July 15, 2008, 01:54:21 PM »
In general, forcing anything through zero is a bad idea. There are zillions of possible reasons that can shift your otherwise perfectly straight line in some direction, so that it doesn't go through 0.
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#### Hunt

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##### Re: 1st order rate constant question
« Reply #2 on: July 16, 2008, 08:17:00 AM »
Someone told me it is better to determine the 1st order rate constant, when measuring reactant A concentration, from ln-([A] - [A]o) vs. t, forcing the intercept to 0.  He said it is wrong to determine from ln[A] vs t because this assumes [A]o is a variable and should not be.  Is he correct?

When you plot Ln([A]) = f(t) , you'll get a straight line with an intercept equal to Ln[A]0 , which is a constant ofcourse and not a variable.

The relation ln-([A] - [A]o) =f(t) doesnt work for a 1st-order reaction.

#### tamim83

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##### Re: 1st order rate constant question
« Reply #3 on: July 16, 2008, 10:12:25 AM »
ln[A]=ln[A0]-kt should though.  k is the rate constant.  So plotting ln[A] vs. t gives you the rate constant (-k=slope) and ln[A0] (the y-intercept).